A. Field of the Invention
This invention pertains generally to raster scanned cathode ray tubes (CRTs) and more particularly to generating a precisely timed blanking pulse in a CRT horizontal deflection circuit.
B. Definitions
Bead inductor means an inductive electronic component that is designed to saturate at a predetermined current level.
Bead transformer means a bead inductor that is configured as a transformer.
Blank means to cause a video image to be substantially not visible on a cathode ray tube screen.
Blanking means to blank or unblank.
Cathode ray tube monitor means a device for viewing a video image using a cathode ray tube device.
Circuit resonance means a resonate frequency oscillation that occurs in a circuit due to the circuit architecture.
Control signal means a signal generated for the purpose of initiating an action or event.
Current path means the route of flow of current in a circuit.
Decode means to identify an outcome based upon an input.
Decoder means a device that decodes.
Detector means a device that senses an input and is capable of generating an output indicating that an input has been sensed.
Direction of current flow changes means a change in current about a zero point.
Electrical pulse means any pulse comprised of current, voltage, energy, or any other measurable electrical component or combination thereof.
Horizontal deflection circuit means circuitry in a cathode ray tube device that is used to control the horizontal deflection of an electron beam.
Horizontal trace means a path of an electron beam across a cathode ray tube that is intended to occur while a video signal is being applied to said electron beam.
Inductor means an inductive electronic component.
Inverter means a device that reverses the polarity of a signal.
Left side ringing means a type of video image distortion characterized by vertical bars at the beginning of a trace of a CRT raster typically on the left side of a CRT screen.
Logic circuit means a device that performs binary operations.
Retrace tuning capacitor means a capacitor that stores energy to create a retrace pulse in a cathode ray tube horizontal deflection circuit.
Switching signals means signals that are capable of turning other circuitry on and off.
Unblank means to not blank.
Video amplifier means a device for amplifying a video signal.
Video image means the displayed image that appears on the cathode ray tube screen which is produced in response to an input video signal.
Video signal means the electronic signal that is applied to the electron guns of a cathode ray tube.
C. Description of the Background.
In cathode ray tube devices, video amplifiers are unblanked during the trace time when the electron beam is tracing the video image (from left to right) and blanked, during the retrace time, while the electron beam is being moved back from right to left, in order to begin the trace of the next image line on the CRT raster. This blanking of the signal amplifiers is done to hide both the horizontal and vertical retrace lines which would be visible in the video image if the amplifiers were not blanked. Also, if the video image is not accurately centered, leaving the video amplifiers unblanked would result in the image being folded or wrapped around the edge of the raster. When horizontal blanking is working well, an off-center picture will be blanked at the edge of the raster and will not be folded back around the CRT screen.
As cathode ray tube devices have become faster and operate at ever higher frequencies, the problem of detecting when, during trace, to blank the video amplifiers and before retrace begins, has become more difficult. This is primarily due to the shorter time period available in which to detect a raster edge and to reverse scan direction. In fact, the duration of time required to switch some semiconductor components commonly used in horizontal deflection circuits is nearly as long as one third of the total trace time. Also the operating characteristics of semiconductor components used in horizontal deflection circuits for switching may vary with environmental conditions further confounding the use of these components for timing.
The usual method of determining the horizontal retrace time is to detect the beginning and end of the horizontal retrace pulse. This pulse is typically 1,000 to 1,500 volts and lasts for the duration of the horizontal retrace. The ending edge of this high voltage pulse is easy to detect because the trailing edge of the pulse falls very quickly, crosses ground (zero volts), and ends slightly below ground. A voltage comparator having a reference voltage set at approximately the ground voltage is capable of detecting the end of the retrace pulse. On the other hand, detecting the start of the retrace pulse or the end of the horizontal trace is very difficult. The difficulty arises because the start of the retrace pulse has a slow rise time and is dependent upon the operating characteristics of the horizontal switching transistor. The operational characteristics of the horizontal switching transistor may vary significantly with time, temperature, and load. Also, because the shape of the voltage curve during the first 10 to 20 volts of the horizontal pulse voltage curve is so unpredictable, the reference voltage input of a voltage comparator used to detect the start of the horizontal retrace pulse must be set high, which causes a time delay in detecting the start of the retrace. This delay adds to the delay of the video amplifier switching circuit, and makes it generally impossible to detect the start of retrace during the current fall time of the horizontal switch transistor. Another problem with the present detection method is that it is difficult and expensive to build a voltage comparator that can handle 1,000 volt pulses and still accurately measure voltage in the range of 10 volts.
In summary, when using the typical voltage detection method for timing CRT horizontal deflection circuits, it is easy to detect the end of a retrace (start of a trace), but difficult to detect the end of a trace (start of retrace). This creates difficulty in determining when to blank the video amplifiers. Often, when using the usual detection method, the horizontal trace reaches the right edge of the raster and has reversed direction before detection is accomplished and before the video amplifiers are blanked. Because of these problems it would, therefore, be desirable to provide an accurate method of determining the beginning of the retrace pulse so that the video amplifiers can be accurately switched at the appropriate times. It is against this background, and the limitations and problems associated therewith, that the present invention has been developed.